Optical fibers are finding increasing applications for information communication and as the area for their use expands, demands are placed upon such optical fibers to withstand the stresses of ever more hostile environments, such as the high temperatures and/or pressures encountered in well logging cables, deep ocean conditions, or chemically corrosive atmospheres. Optical fibers are thus being demanded that exhibit higher durability both in strength as well as in protection against environmental conditions.
Various coatings have been proposed as additions to optical fibers aimed at providing an increase in their durability and environmental protection. Among these are layers of metals such as aluminum, indium, tin, gold, tungsten, titanium, zirconium, molybdenum, and metal alloys. Such metal coatings, however, have the undesirable property of conducting both heat and electricity and do not provide adequate long term protection against hostile environments, such as those represented by the well logging and deep ocean applications.
Plastic coatings including those of kevlar, teflon, ultra-violet setting epoxies, laquers, and nylon have also been used for optical fiber coatings. Such plastics alone are inadequate as barriers against water penetration under conditions of high pressure or temperature.
Ceramic coatings have also been proposed for optical fibers particularly to improve the fiber tensile strength. These ceramics, typically applied in on-line processing, also pose substantial difficiencies. Among these is a tendency of such ceramic coatings to dissolve at elevated temperatures and pressures in the presence of acids and bases. In addition, ceramic coatings often possess expansion coefficient which mismatch those of the fiber so that cracks in the coating develop, particularly from repeated temperature or pressure cycling.
An additional problem encountered with optical fiber coatings in general is that they lead to reduced adhesion between the fiber coating and subsequently applied plastic buffers, whether applied over ceramic, metllic or nonmetallic coatings. This low adhesion is not always desirable, since it eventually will lead to the creation of pockets or bubbles between the fiber and the buffer where adhesion has been lost between those layers.